Which version of twrp are you using?I'm using v.126.96.36.199 and there's an option in there to mount sd card. Once you do that it'll open up on your pc and you'll be able to drag and drop the ROM on your sd card. At least that's what worked for me.
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?